Candidemia is the 4th most common nosocomial infection, and leads to disproportionate mortality. Candida albicans is a ubiquitous commensal and the most frequent cause of fungal disease in humans. Disseminated bloodstream candidemia carries a >30% attributable mortality and is associated with chemotherapy-induced neutropenia, organ transplantation, primary genetic defects such as chronic granulomatous disease, and medical procedures such as intravenous catheterization that provide direct access to the bloodstream. Human epidemiology and mouse experiments have demonstrated that innate immune function is the primary barrier against disseminated disease, yet the role of macrophages in response and clearance of disease is still unclear. Unfortunately, technical constraints have limited our ability to test the roles of macrophages in their natural context of infection. The relatively weak capacity of isolated macrophages to destroy C. albicans in the absence of matrix and soluble cues has led to the idea that they play a marginal role limiting infection but could promote dissemination of disease among tissues. However, our recent non-invasive imaging, using a new transparent model of disseminated Candida-zebrafish infection, indicates that macrophages in their natural context have an enhanced ability to limit proliferation and germination of C. albicans. We hypothesize that macrophages limit filamentous growth of C. albicans but promote dissemination of infection throughout the body. We will exploit the zebrafish infection model to determine the role of macrophages in limiting fungal proliferation and promotion of dissemination using intravital real-time imaging, gene knockdown, targeted cell ablation, and fungal mutants. Transgenic fish with marked macrophages will be used to follow individual fungal-phagocyte interactions in vivo, and we will ablate and perturb macrophages to determine their role in regulating fungal morphogenetic switching, proliferation and dissemination. The proposed experiments will test basic questions of innate immunity that have been intractable in mammalian models by exploiting the advantages of the transparent zebrafish model. The results of the proposed experiments will yield a more complete basic understanding of the mechanisms underlying innate immunity to candidemia and set the stage for targeted screening for novel host and pathogen pathways regulating these macrophage activities. The discovery of mechanisms underlying macrophage activity during fungal infection has the potential to lead to new therapeutic interventions. PUBLIC HEALTH RELEVANCE: Infectious diseases affect humans worldwide and cause significant mortality and morbidity in the United States. Current methods to control fungal disease are inadequate, especially in the immunocompromised patient population, so it is critical to investigate alternative treatments. A long-term goal of this investigation involves leveraging basic information about host-pathogen interaction to develop therapeutics to treat fungal disease.